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1.
High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.  相似文献   

2.
Garnet–clinopyroxene intermediate granulites occur as thin layers within garnet–kyanite–K–feldspar felsic granulites of the St. Leonhard granulite body in the Bohemian Massif. They consist of several domains. One domain consists of coarser‐grained coexisting ternary feldspar, clinopyroxene, garnet, quartz and accessory rutile and zircon. The garnet has 16–20% grossular, and the clinopyroxene has 9% jadeite and contains orthopyroxene exsolution lamellae. Reintegrated ternary feldspar and the Zr‐in‐rutile thermometer give temperatures higher than 950 °C. Mineral equilibria modelling suggests crystallization at 14 kbar. The occurrence and preservation of this mineral assemblage is consistent with crystallization from hot dry melt. Between these domains is a finer‐grained deformed matrix made up of diopsidic clinopyroxene, orthopyroxene, plagioclase and K‐feldspar, apparently produced by reworking of the coarser‐grained domains. Embedded in this matrix, and pre‐dating the reworking deformation, are garnet porphyroblasts that contain clinopyroxene, feldspar, quartz, rutile and zircon inclusions. In contrast with the garnet in the coarser‐grained domains, the garnet generally has >30% grossular, the included clinopyroxene has 7–27% jadeite and the Zr content of rutile indicates much lower temperatures. Some of these high‐grossular garnet show zoning in Fe/(Fe + Mg), decreasing from 0.7 in the core to 0.6 and then increasing to 0.7 at the rim. These garnet are enigmatic, but with reference to appropriate pseudosections are consistent with localized new mineral growth from 650 to 850 °C and 10 to 17 kbar, or with equilibration at 20 kbar and 770 °C, modified by two‐stage diffusional re‐equilibration of rims, at 10–15 and 8 kbar. The strong pervasive deformation has obscured relationships that might have aided the interpretation of the origin of these porphyroblasts. The evolution of these rocks is consistent with formation by igneous crystallization and subsequent metamorphism to high‐T and high‐P, rather than an origin by ultrahigh‐T metamorphism. Regarding the petrographic complexity, combination of the high grossular garnet with the ternary feldspar to infer ultrahigh‐T metamorphism at high pressure is not justified.  相似文献   

3.
High‐pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust. Orthopyroxene‐free assemblages in granitic (ternary feldspar(s) + quartz + garnet + kyanite + rutile), intermediate (ternary feldspar(s) + quartz + garnet + clinopyroxene ± kyanite ±rutile ± titanite) and basic (garnet + clinopyroxene + plagioclase ± quartz + rutile) compositions indicate formation conditions at mantle depths. Clinopyroxene compositions in Variscan high‐pressure granulites are unusual in that they include omphacite (in plagioclase‐bearing rocks thus not eclogite) and Al‐rich diopside (i.e. indicating high Ca‐Tschermak content), with both yielding temperatures above 900 °C. Problems such as compositional zoning, multiple generations of key phases in reaction domains and unmixing of high‐temperature solid‐solution phases during cooling (ternary feldspars, omphacite) clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association. Pressure–temperature (P–T) determination by the pseudosection method, although allowing prediction of mineral assemblages, compositions and molar proportions for a fixed bulk composition for modelled P–T conditions, still requires reliable activity–composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples, i.e. the same restrictions applying to conventional thermobarometry. The interpretations of some recently determined pseudosections for the composition of Variscan clinopyroxene‐bearing high‐pressure granulites contradict numerous published P–T paths. However, quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations. In the case of the Variscan example, it is argued that petrographic observations and published P–T paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models.  相似文献   

4.
The granulites of the Saxon Granulite Massif equilibrated athigh pressure and ultrahigh temperature and were exhumed inlarge part under near-isothermal decompression. This raisesthe question of whether P–T–t data on the peak metamorphismmay still be retrieved with confidence. Felsic and mafic granuliteswith geochronologically useful major and accessory phases haveprovided a basis to relate P–T estimates with isotopicages presented in a companion paper. The assemblage garnet +clinopyroxene in mafic granulite records peak temperatures of1010–1060°C, consistent with minimum estimates ofaround 967°C and 22·3 kbar obtained from the assemblagegarnet + kyanite + ternary feldspar + quartz in felsic granulite.Multiple partial overprint of these assemblages reflects a clockwiseP–T evolution. Garnet and kyanite in the felsic granulitewere successively overgrown by plagioclase, spinel + plagioclase,sapphirine + plagioclase, and biotite + plagioclase. Most ofthis overprinting occurred within the stability field of sillimanite.Garnet + clinopyroxene in the mafic granulite were replacedby clinopyroxene + amphibole + plagioclase + magnetite. Thehigh P–T conditions and the absence of thermal relaxationfeatures in these granulites require a short-lived metamorphismwith rapid exhumation. The ages of peak metamorphism (342 Ma)and shallow-level granitoid intrusions (333 Ma) constrain thetime span for the exhumation of the Saxon granulites to  相似文献   

5.
Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.  相似文献   

6.
Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (XMg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (XMg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of XMg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.  相似文献   

7.
High‐pressure (HP) granulites and eclogitized metagabbro are exposed along an orogen‐parallel high‐P belt that was developed at c. 1050–1020 Ma in the NE Grenville Province. Among these rocks, mafic granulites derived from a Labradorian anorthosite suite of the Lelukuau terrane contain garnet, Al‐Na diopside, and, depending on bulk composition, plagioclase and kyanite. Moreover, the distribution of phases is influenced by the original igneous texture. For instance, in high XMgO leucocratic varieties, garnet porphyroblasts nucleated together with kyanite in An‐rich cores of plagioclase domains whereas in low XMgO rocks garnet occurs together with clinopyroxene within formerly igneous ferromagnesian domains and kyanite is missing. In contrast, garnet pseudomorphs after igneous plagioclase in melanocratic varieties display evidence of earlier corona development. Metamorphic textures are consistent with a two stage evolution: (a) development of garnet and Al‐Na‐diopside (Cpx1) under high‐P metamorphic conditions, concomitant with elimination of plagioclase in the mesocratic to melanocratic varieties; and (b) partial loss of Al‐Na from Cpx1 resulting in production of new andesitic plagioclase, and growth of new clinopyroxene (Cpx2) after garnet and quartz in leucocratic to mesocratic rocks consistent with decompression. Widespread equilibrium textures between garnet‐Pl2‐Cpx2 and/or reset Cpx1 are consistent with development at the thermal peak. Estimated P–T conditions for the presumed thermal peak fall in the range 1500–1800 MPa and 800–900 °C and are comparable to those recorded by eclogitized gabbros from other parts of the high‐P belt of the NE Grenville province. Low jadeite content of clinopyroxene from the HP granulites is attributed to the low bulk Na2O/(Na2O + CaO) of these rocks relative to common basaltic compositions. Scarcity of apparent retrograde textural overprint in both the HP granulites and the eclogites suggests fast subsequent cooling, consistent with extrusion of the high‐P belt towards the foreland shortly after the metamorphic peak.  相似文献   

8.
Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.  相似文献   

9.
High‐pressure granulites are generally characterized by the absence of orthopyroxene. However, orthopyroxene is reported in a few high‐pressure, felsic–metapelitic granulites, such as the Huangtuling felsic high‐pressure granulite in the North Dabie metamorphic core complex in east‐central China, which rarely preserves the high‐pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K‐feldspar + quartz. To investigate the effects of bulk‐rock composition on the stability of orthopyroxene‐bearing, high‐pressure granulite facies assemblages in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system, a series of P–T–X pseudosections based on the melt‐reintegrated composition of the Huangtuling felsic high‐pressure granulite were constructed. Calculations demonstrate that the orthopyroxene‐bearing, high‐pressure granulite facies assemblages are restricted to low XAl [Al2O3/(Na2O + CaO + K2O + FeO + MgO + Al2O3) < 0.35, mole proportion] or high XMg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. This study also reveals that the XAl values in the residual felsic–metapelitic, high‐pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene‐bearing, high‐pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.  相似文献   

10.
A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.  相似文献   

11.
High-pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging-wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution.  相似文献   

12.
New data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwise P–T path is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages of c. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.  相似文献   

13.
A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H2O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H2O=hornblende+quartz and plagioclase+hornblende+H2O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.  相似文献   

14.
A re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ～16 kbar and 750 °C, followed by near‐isothermal decompression to ～10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ～7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ～25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ～2–5 mm year?1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.  相似文献   

15.
Felsic to mafic granulite xenoliths from late Neogene basalt pyroclastics in four localities of the western Pannonian Basin (Beistein, Kapfenstein, Szigliget and Káptalantóti (Sabar-hegy) were studied to find out their metamorphic and fluid history. The characteristic mineral assemblage of the granulites consists of Pl + Opx + Qtz ± Cpx ± Bt ± Grt ± Kfs. Based on abundant magmatic relic microstructural domains occurring in these rocks, the potential precursors might have been predominantly felsic igneous or high to ultrahigh temperature rocks. Ternary feldspar thermometry provides a rough estimate of temperatures of about 920–1070 °C. The first fluid invasion event, which is linked with this early high to ultrahigh temperature stage is characterised by primary pure CO2 inclusions in apatite and zircon. The densest primary CO2 inclusions indicate 0.52–0.64 GPa pressure at the estimated temperature range of crystallization. According to mineral equilibria and geothermobarometry, the high to ultrahigh temperature rock cooled and crystallized to granulite of predominantly felsic composition at about 750–870 °C and 0.50–0.75 GPa in the middle crust, between 20 and 29 km depths. The second fluid invasion event is recorded by primary CO2-rich fluid inclusions hosted in the granulitic mineral assemblage (plagioclase, quartz and orthopyroxene). In addition to CO2, Raman spectroscopy revealed the presence of minor N2, H2S, CO and H2O in these inclusions. Partial melting of biotite-bearing assemblages could be connected to the next fluid invasion shown by secondary CO2-rich fluids recorded along with healed fractures in plagioclase, clinopyroxene and orthopyroxene. This event could have happened at depths similar to the previous ones. The final step in the granulite evolution was the sampling in the middle crust and transportation to the surface in form of xenoliths by mafic melt. This event generated temperature increase and pressure decrease and thus, limited melting of the xenoliths. The youngest fluid inclusion generation, observed mostly in healed fractures of felsic minerals, could be associated with this event.  相似文献   

16.
Monazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are consistent with the garnet‐rich rocks representing the residual products of ultrahigh temperature melting of biotite‐bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64–2.61 Ga Y‐rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium‐poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid‐absent melting of biotite + plagioclase + quartz ± sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61–2.55 Ga Y‐depleted, Th‐rich monazite domains crystallized in the presence of garnet + ternary feldspar ± orthopyroxene + peraluminous melt. The c. 2.58–2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high‐pressure (HP) grossular‐rich garnet at the expense of plagioclase during crustal thickening, culminating at >950 °C. Re‐heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn‐kinematic growth of (La + Ce)‐enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0–1.2 GPa and 600–700 °C. Monazite grains in this study are marked by positive Eu‐anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu‐anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu‐enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.  相似文献   

17.
变质程度达角闪岩相及麻粒岩相的各类变质岩石在变质作用高峰期后，如果降温速率比较慢，往往发生退变质性质的离子再交换反应与纯转换反应。这两类退变质反应不可避免地改造了变质高峰期的矿物成分，使岩石不能准确记录变质作用各阶段的矿物成分和变质作用p-T轨迹。对这两类反应的研究已成为近年来变质岩石学研究的热点之一。本文通过研究实例，简要综述了基性麻粒岩中石榴子石-斜方辉石之间的Fe-Mg再交换反应、酸性麻粒岩中斜长石-碱性长石之间的K-Na再交换反应、泥质片麻岩中石榴子石变斑晶边部的部分分解(纯转换)反应，以及恢复变质高峰期矿物成分和计算变质高峰期p-T条件的方法。指出只有在准确恢复变质高峰期变质矿物成分的基础上，获得的变质作用p-T-t轨迹才能反映客观地质事实。  相似文献   

18.
The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.  相似文献   

19.
High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.  相似文献   

20.
The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (XAn = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (XJd = 0.20–0.15), and TiO2 in amphibole (TiM2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (XAn = 0.25–0.32), and grossular content in garnet (XGrs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (TiM2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.  相似文献   

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